I have found small black snails that resemble these (NZ Mud Snails) in one of my favorite tidal creeks that flows into Willapa Bay. However, the ones I've found seem to attain a larger size, up to anywhere between 1/3" and 1/2" in length. Lots of smaller ones, though.
Are there indigenous snails that resemble the New Zealand Mudsnail in appearance, but are usually larger, and black?

edit: Jeremy, just saw the photo you posted. Hmmmm, the ones I am seeing are more conical in shape than those, and closely resemble the pics I've seen of NZMS, except the ones in lower Smith Creek are larger and almost black.

So I would like to know if there is an indigenous freshwater snail found in streams along the coast that fits that description. My searches have found a lack of info containing any descriptions that include line drawings or photographs on this subject. I am having difficulty hacking thru the taxonomic jungle.

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Me too its crazy how many species of snails there are these are mostly like 2mm to 4mm their tiny but i cant find anything on them and the ones that resemble them are the NZMS

If as Skeena88 stated,these snails have been found in Capital Lake and have probably been there for a while,I would assume it would be a simple matter for them to migrate or be carried to the upper Deschutes river. Question for you biologist folks,do they take over the habitat to a point where they limit the natural insects survival?

The snails do occupy a large area of stream bed and can be found at densities of >500,000 critters per square meter. I have some papers at work that discuss effects of NZ mud snails on native invertebrates, but at that density they can significantly displace and compete with native bugs, so a reduction in invertebrate diversity is inevitable.
There was a study that looked at water bodies in a specified radius (don't recall the distance but it wasn't small) around Capitol Lake and didn't find any snails. The snails can migrate upstream slowly at about 800 meters/yr in low gradient systems, but the colonization pattern isn't usually a spreading radius around a single focal point which would indicate a gradual expansion of a local population, but rather is sporadic and wide ranging which usually means that the bugs have jumped large distances by being transported by a host. The host could be boats, wading gear, animals etc.

NZM are sensitive to cold weather, so they drained Capitol lake a few years ago in the winter to let the cold air kill the snails; it didn't get all of them, but apparently it did knock the population back. Of course, you can't drain the Madison river in the winter...

The snails graze on microscopic algae that form the base of the food chain in freshwater ecosystems. At high density they can outcompete the native invertebrates for food, thereby truncating the food chain that trout rely upon.

Thanks for that map, Richard. I clicked on your link and zoomed in on the mouth of the Columbia R and zeroed in on the WA side. Looks like one was found at the boat launch in Ilwaco, and other ones found at a couple of other spots along the shoreline and jetties. Actually, it looks like 3 - 5 were found at the boat launch over near the Coast Guard Station. The color code for the dots indicates only one snail found at each other spot, but it does indicated their presence. These could have arrive on waders, boots, maybe boats/trailers, shore birds or migrating water fowl, etc.
I didn't see any locations "around the corner" in Willapa Bay, but I did see a couple dots on the Grays River, which I have fished in waders and felt soles, but not for 3 or 4 years.
My guess is that we'll likely see them in Willapa Bay streams very shortly. I'll bet that they are already in the Naselle River. Its not that far from the Grays. Lots of fishers can hit each river system in the same day, if they want to.

I guided the madison for the years when the NZsnails began their takeover. Towards the end of my time there you could feel them crunching underfoot as you waded in some sections. I have always wondered if the fish ate them although I doubted it. Some of the sections were just black on the bottom.I haven't been back for years so I don't know what its like now.I don't know if they have made it to the Henry's Fork and Gallatin rivers but probably so. Back then we didn't have the knowledge of what was going on like exists now. Since I guided all the rivers I probably spread them around myself.It's to bad that by the time you figure out how serious an invasive species is,it's to late!

You will still find lots of folks who apparently aren't aware of the issues associated with transporting stuff from one body of water to another. I still see boat trailers going down the road trailing all kinds of aquatic vegetation. Unbelievable, but true. Mainly locals, here in my area, that I observe doing that. I might just start calling them in, if I can get their license #'s. Just a vehicle description might suffice..

Jeremy, the ones in your photos don't look as "conical" or tapered as the ones in the file photos of NZ Mudsnails, as far as I can tell. Count the number of whorls on the shells. NZMS have 5 whorls.
I don't see five on your samples.

This is a problem that is not going to go away any time soon. Having grown up in the northeast myself, I witnessed first-hand the incredible explosion of the zebra mussel and other invasive species on our Great Lakes waters and beyond. This is something I would like to see more anglers take on as volunteers, possibly coordinating between wdfw and the Washington State University Water Programs.http://olympicpeninsulaflyfishing.blogspot.com

I believe your freshwater snails to be of family Hydrobiidae (Pebble Snails) and of genus Fluminicola. Please see below image:

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thank you very much that is what i was looking for I know its a gastropod and it is a Lithoglyphinae but then from there its tricky there is so much to snails aand so many species even if their not invasive its good to know what part they play in my local river